The resource list below provides links to digital images
of original 18th and early 19th-century dictionaries and Spanish grammar
texts, focusing on those that will assist English-speakers in reading and
understanding historical documents from the Spanish colonial era, including
those that were written in ecclesiastical Latin. In
addition, these texts provide an opportunity for even native
Spanish-speakers to explore the nuances of linguistic change that have
occurred during the past three centuries, most notably in the area of
vocabulary, including many words no longer in use.
Apart from the searchable index of Spanish-Spanish dictionaries (below), all other
texts are freely available on
Google Books,
with a few from the
Biblioteca
Digital Hispánica. A wide range of colonial-era book scans are
freely available and internally searchable via
Google Books; the
following links can be used as a starting point for other searches (search
settings are for free books in the indicated language dating before 1850):
Spanish,
English, and
French.

1611 -
Tesoro de la Lengua Castellana, o Española, by Sebastián de Cobarruvias
Orozco (selection of Spanish words with meanings and brief discussions of their
origins and usage; equivalent to a limited etymological dictionary)

1726-present - All comprehensive historical Spanish dictionaries are conveniently searchable
with digital images of the original pages at the
Nuevo Tesoro Lexografico de la Lengua Espanola
(click search button at top left, then enter search term in box entitled "Lema")